linux-kernel - Re: [PATCH] nvmem: fix unregistering device in nvmem

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Date:   Wed, 22 Dec 2021 10:34:42 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Johan Hovold <johan@...nel.org>
Cc:     Rafał Miłecki <zajec5@...il.com>,
        Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
        Andrey Smirnov <andrew.smirnov@...il.com>,
        linux-kernel@...r.kernel.org,
        Rafał Miłecki <rafal@...ecki.pl>
Subject: Re: [PATCH] nvmem: fix unregistering device in nvmem_register()
 error path

On Wed, Dec 22, 2021 at 10:24:33AM +0100, Johan Hovold wrote:
> On Wed, Dec 22, 2021 at 10:03:17AM +0100, Johan Hovold wrote:
> > On Wed, Dec 22, 2021 at 09:56:29AM +0100, Greg Kroah-Hartman wrote:
> > > On Wed, Dec 22, 2021 at 09:38:27AM +0100, Johan Hovold wrote:
> > > > On Wed, Dec 22, 2021 at 08:44:44AM +0100, Greg Kroah-Hartman wrote:
> > > > > On Tue, Dec 21, 2021 at 06:46:01PM +0100, Rafał Miłecki wrote:
> > > > > > On 21.12.2021 17:06, Greg Kroah-Hartman wrote:
> > > > > > > On Tue, Dec 21, 2021 at 04:45:50PM +0100, Rafał Miłecki wrote:
> > > > > > > > From: Rafał Miłecki <rafal@...ecki.pl>
> > > > > > > > 
> > > > > > > > 1. Drop incorrect put_device() calls
> > > > > > > > 
> > > > > > > > If device_register() fails then underlaying device_add() takes care of
> > > > > > > > calling put_device() if needed. There is no need to do that in a driver.
> > > > > > > 
> > > > > > > Did you read the documentation for device_register() that says:
> > > > > > > 
> > > > > > >   * NOTE: _Never_ directly free @dev after calling this function, even
> > > > > > >   * if it returned an error! Always use put_device() to give up the
> > > > > > >   * reference initialized in this function instead.
> > > > > > 
> > > > > > I clearly tried to be too smart and ignored documentation.
> > > > > > 
> > > > > > I'd say device_add() behaviour is rather uncommon and a bit unintuitive.
> > > > > > Most kernel functions are safe to assume to do nothing that requires
> > > > > > cleanup if they fail.
> > > > > > 
> > > > > > E.g. if I call platform_device_register() and it fails I don't need to
> > > > > > call anything like platform_device_put(). I just free previously
> > > > > > allocated memory.
> > > > > 
> > > > > And that is wrong.
> > > > 
> > > > It seems Rafał is mistaken here too; you certainly need to call
> > > > platform_device_put() if platform_device_register() fail, even if many
> > > > current users do appear to get this wrong.
> > > 
> > > A short search found almost everyone getting this wrong.  Arguably
> > > platform_device_register() can clean up properly on its own if we want
> > > it to do so.  Will take a lot of auditing of the current codebase first
> > > to see if it's safe...
> > 
> > Right, but I found at least a couple of callers getting it it right, so
> > changing the behaviour now risks introducing a double free (which is
> > worse than a memleak on registration failure). But yeah, a careful
> > review might suffice.
> 
> Actually, I'm not sure we can (should) change
> platform_device_register(). The platform device has been allocated by
> the caller and it would be quite counterintuitive to have the
> registration function deallocate that memory if registration fails.
> 
> Heh, we even have statically allocated structures being registered with
> this function and we certainly don't want the helper to try to free
> those.

Yeah, it's a mess.  I'll try to look at it this break if things calm
down...

greg k-h